Aggregation of Blood Pressure in the Families of Identical Twins

The multiple relationships which exist within families of adult monozygotic (MZ) twins permit evaluation of genetic and environmental variance and provide a direct test of maternal effects. Systolic blood pressure (SBP) was measured in 610 members of the families of 76 MZ twin pairs. Age and sex effects on BP were removed via a Z-transformation. Correlations for SBP were .72 for MZ twins and .23 for full siblings. Regression of offspring on single parent was .26. The husband-wife correlation was significant, but it was no higher than the resemblance of a twin to the spouse of his co-twin. Comparisons of maternal and paternal half-siblings provide no evidence of maternal influences on SBP. To evaluate whether familial resemblance of BP is mediated by body size, the original systolic pressures were adjusted by stepwise multiple regression of age, sex, height, weight and skinfold thickness. Adjusting BP for effects of body size reduced regression of offspring on mid-parent from .40 to .23. The authors conclude that familial aggregation of systolic pressure reflects additive genetic variance mediated, in part, by body size and augmented by social homogamy arising from non-random mating.